ABSTRACT This fi eld trip guide explores the interactions among the geologic evolution, hydrology, and fluvial geomorphology of the central Oregon Cascade Range. Key topics include the geologic control of hydrologic regimes on both the wet and dry sides of the Cascade Range crest, groundwater dynamics and interaction between surface and groundwater in young volcanic arcs, and interactions between rivers and lava flows. As we trace the Willamette and McKenzie Rivers back to source springs high in the young volcanic rocks of the Cascade Range, there is abundant evidence for the large permeability of young lava flows, as manifested in streams that dewater into lava flows, lava-dammed lakes in closed basins, and rivers that emerge from single springs. These dynamics contrast sharply with the older, lower permeability Western Cascades terrane and associated runoff-dominated fluvial systems. On the east side of the Cascades we encounter similar hydrologic characteristics resulting in complex interactions between surface water and groundwater as we follow the Deschutes River downstream to its confluence with the Crooked River. Here, deep canyons have cut through most of the permeable part of the geologic section, have been invaded by multiple large intracanyon lava flows, and are the locus of substantial regional groundwater discharge. The groundwater and surface-water interaction in the Deschutes Basin is further complicated by surface-water diversions and an extensive network of leaking irrigation canals. Our west-to-east transect offers an unparalleled opportunity to examine the co-evolution of the geology and hydrology of an active volcanic arc.

Abstract Since the days of John Muir, the striking granitic topography of Yosemite Valley, California, has been understood to have been sculpted by glaciers and presently modified by rockfall. Glacial erosion has provided remarkably clean and extensive exposures of granitic rocks on the vertical walls that provide insights into intrusive relations and rockfall susceptibility. However, it is only with recent remote sensing methods that these exposures have been studied in detail. El Capitan presents an unparalleled exposure of the interior of a granitic plutonic system at the point of interaction between multiple intrusive suites and two sets of mafic dike swarms. The distribution and orientation of these units affected El Capitan's extensive rockfall history, including a huge postglacial rock avalanche at 3.6 ka. This two-day field trip will explore these ideas and apply them to some of the other classic cliffs of Yosemite Valley such as Glacier Point and Half Dome. We will present a new map of El Capitan and discuss the intrusive relationships exposed on the face while visiting several rockfall deposits and some of the classic vistas of Yosemite Valley, including El Capitan Meadow, Glacier Point, Taft Point, and Mirror Lake.